This invention relates to electrophysiology; and, more particularly, to a system and method for utilizing two or more physiologic voltage potential signals to determine the voltage potential that would be measured between a virtual electrode pair placed at a predetermined location on, or within, a body.
At any given point in time, a selected point within a living body may be at a different voltage potential than that of another selected point in the body. Moreover, the voltage potential at a given point is likely changing with time. Electrodes positioned at two distinct points will therefore measure a potential difference signal between those two points that is varying over time.
A common example of a system of measuring potential differences within the human body is provided by the electrocardiogram (ECG), which refers to a plot against time of the varying potential differences existing between various standard electrode pairs positioned on the surface of the body. A conventional ECG measurement will include twelve signal measurements, also referred to as xe2x80x9cleadsxe2x80x9d, that are taken using a set of standard electrodes pairs.
FIG. 1 illustrates the ten standard electrode positions used to obtain a twelve-lead ECG measurement. Electrodes RA, LA, and LL are positioned on a patient""s right arm, left arm, and left leg respectively, and a ground is generally placed on the right leg (RL). Other electrodes V1 through V6 are placed on the patient""s chest. Various electrode pairs are used to obtain the standard set of twelve leads included in an ECG measurement.
Three of the signals measured are commonly referred to as Lead I, Lead II, and Lead III. These refer to measurements between RA and LA, between RA and LL, and between LA and LL, respectively. These three signal measurements comprise what is called Einthoven""s triangle, shown in FIG. 2. This triangle is commonly used to show the relationship between the measured electrical signals and the lead positions. This can be expressed in equation form as follows:
lead II=leadI+leadIII.
This concept is based on Kirchoff""s voltage law, wherein the voltage signals as measured between the right to left arm, between left arm to left leg, and between left leg to right arm may be added to obtain a sum of zero if the first point in each pair is considered the voltage reference point. As is evident from the foregoing equation, any one of the signals of Einthoven""s triangle may be approximated if the other two signals are known. By extending this concept, all of the signals included in the standard 12-lead ECG may be approximated if only two of the signals are known.
Systems have been developed to utilize ones of the 12-signal ECG measurements to derive other measurements. For example, U.S. Pat. No. 5,231,990 to Gauglitz describes a circuit that adds various ones of the standard ECG signals to generate other ones of the standard ECG signals. A similar system is described in U.S. Pat. No. 5,711,304 to Dower which discloses using ones of the 12-lead ECG signals to calculate the signals that exist at predetermined non-standard ECG positions on a body. U.S. Pat. No. 4,023,565 issued to Ohlsson describes circuitry for recording ECG signals from multiple lead inputs. Similarly, U.S. Pat. No. 4,263,919 issued to Levin, U.S. Pat. No. 4,170,227 issued to Feldman, et al, and U.S. Pat. No. 4,593,702 issued to Kepski, et al, describe multiple electrode systems that combine surface ECG signals for artifact rejection.
U.S. Pat. No. 6,038,469 to Karlsson et al. discloses a cardiac monitoring system that continuously stores three perpendicular leads X, Y, and Z, and derives a standard 12-signal ECG signal there from in real time. Another similar system is described in U.S. Pat. No. 4,850,370 to Dower, which discloses the use of four electrode positions on the chest of a patient to derive xyz vector cardiographic signals or the standard 12-lead ECG signal set. U.S. Pat. No. 5,366,687 to Evan et al. discloses the use of a standard 10-electrode ECG configuration to derive a spatial distribution of signals representative that would be collected from a system having 192 electrodes.
Numerous body surface ECG monitoring electrode systems have been employed in the past in detecting the ECG and conducting vector cardiographic studies. For example, U.S. Pat. No. 4,082,086 issued to Page, et al., discloses a four electrode orthogonal array that may be applied to the patient""s skin both for convenience and to ensure the precise orientation of one electrode to the other. U.S. Pat. No. 3,983,867 issued to Case describes a vector cardiography system employing ECG electrodes disposed on the patient in normal locations and a hex axial reference system orthogonal display for displaying ECG signals of voltage versus time generated across sampled bipolar electrode pairs.
The above-described systems utilize standard ECG measurements to derive other standard measurements. Similar techniques may be performed using Subcutaneous Electrode Arrays (SEAs) located under a patient""s skin. U.S. Pat. No. 5,331,966 issued to Bennett, incorporated herein by reference, discloses a method and apparatus for providing an enhanced capability of detecting and gathering electrical cardiac signals via an array of relatively closely spaced subcutaneous electrodes located on the surface of an implanted device. More recently, U.S. patent application Ser. No. 09/697,438 filed Oct. 26, 2000 entitled xe2x80x9cSurround Shroud Connector and Electrode Housings for a Subcutaneous Electrode Array and Leadless ECGsxe2x80x9d, by Ceballos, et al., incorporated herein by reference in its totality, discloses an alternate method and apparatus for detecting electrical cardiac signals via an array of subcutaneous electrodes located on a shroud circumferentially placed on the perimeter of an implanted pacemaker. An associated U.S. patent application Ser. No. 09/703,152 filed Oct. 31, 2000 entitled xe2x80x9cSubcutaneous Electrode for Sensing Electrical Signals of the Heartxe2x80x9d by Brabec et al., incorporated herein by reference in its totality, discloses the use of a spiral electrode using in conjunction with the shroud described in the ""438 application. In addition, U.S. patent application Ser. No. 09/696,365 filed Oct. 25, 2000 entitled xe2x80x9cMultilayer Ceramic Electrodes for Sensing Cardiac Depolarization Signalsxe2x80x9d, by Guck et al, disclosed the use of multi-layer ceramic electrodes placed into recesses incorporated along and into the peripheral edge of the implantable pacemaker.
As discussed above, both ECG leads and SEA arrays may be used to derive standard measurements. However, standard measurements may not always provide the desired diagnostic information. In some situations, non-standard signals provide more relevant information than is provided by a standard set of signals. For example, in situations in which the optimal angle of placement for an implantable device is to be determined, it is desirable to calculate voltage differences at all possible angles of implant using an electrode spacing that approximates the spacing of electrode pairs as they will exist after implant. What is needed, therefore, is a system and method that extends the prior art concepts to provide a system that utilizes non-standard measurements including measurements derived from implanted electrodes, to automatically generate an infinite number of other non-standard measurements.